def check_faild_exams(sudents, first_try, second_try_student):
    return set(sudents) - set(first_try) - set(second_try_student)

assert check_faild_exams(["T1", "T2", "T3", "T4", "T5", "T6", "T7", "T8", "T9"],
    ["T4","T5", "T6"],
    ["T7", "T8", "T9"]) == {"T1", "T2", "T3"}